For sheets and films, a number of investigators have used nanotubes as the reinforcement in a nanotube/polymer composite. For example, Schadler et al. (1998) and Gong et al. (2000) produced composites of nanotubes in epoxy. Shaffer and Windle (1999) examined a nanotube/poly(vinyl alcohol) composite. Bower et al. (1999) fabricated a composite with nanotubes in a polyhydroxyaminoether. Unless sheets and films are unusually thin, they can also be reinforced with more normally-sized (e.g., diameters of 100 microns or more) reinforcement. However, nano-scale reinforcement is uniquely suited for strengthening polymer fibers, since the fibers themselves are typically only 10 to 100 microns in diameter. Since nanotubes are orders of magnitude smaller in diameter, a nanotube cannot occlude a high fraction of the fiber cross-section.
Presently, fibers produced from “commodity” polymers (e.g., polyester, polypropylene, and nylon) have tensile strengths from about 0.15 to 0.6 Gpa. More expensive “specialty” fibers (such as Kevlar® and PAN carbon fiber) have strengths of about 2 to 5 Gpa. The recently discovered carbon nanotubes have a theoretical strength of 200 Gpa (Schadler et al., 1998)—about 40 times higher than existing materials. However, capitalizing on this potential strength has thus far been problematic.
Several research teams have used single walled carbon nanotubes (SWNTs) to enhance the strength of neat fibers. Andrews et al. (1999) dispersed SWNTs in isotropic petroleum pitch. With a 5 wt % loading, the tensile strength and modulus were increased 90 and 150%, respectively. Haggenmueller et al. (2000) reinforced PMMA (polymethyl methacrylate) with SWNTs. They found a 54% increase in tensile strength and a 94% increase in modulus when an 8 wt % loading of nanotubes was used.
Most multiwalled carbon nanotubes (MWNTs) are believed to have the “Russian doll” structure where only weak van de Waal forces bond one tube to another (Harris, 1999). Hence, the outer layers of MWNTs could slide or telescope relative to each other (Schadler et al., 1998; Shaffer and Windle, 1999). However, kinks and defects could help prevent this sliding (Harris, 1999). Ruoff and Lorents (1996) believe that SWNTs are preferable to MWNTs because SWNTs are easier to bond than MWNTs. These researchers also feel that the tensile strength of the modified SWNTs might be affected by bonding. However, Garg and Sinnott (1998; also see Harris, 1999) showed in theoretical calculations that covalent attachments only decrease SWNT strength by about 15%.
Carbolex® AP Grade Nanotubes is a type of commercially available nanotube material. Carbolex® AP Grade Nanotubes is an “as prepared” nanotube material that contains about 70% SWNT and is produced by a carbon arc process. Previous investigators have used ultrasonic mixing (Schadler et al., 1998) or mechanical mixing and a surfactant (Gong et al., 2000) in attempts to disperse material of this type. However, both of these investigative teams reported that dispersion was not uniform, and that further work was needed. With the unique size range of nanotubes, the phase behavior of nanotubes in polymers will probably affect their dispersion. For submicron particles, phase separation processes occur which are not observed in macroscopic (micron-scale) systems. In particular, colloidal crystals are produced which depend on the form of interparticle forces (Calvert, 1997; Milling et al., 1991; Vincent, 1987).
Melt spinning involves polymers that are melt processable (thermoplastic). The polymer is melted, pressurized, and forced through a fine capillary. The fiber can be drawn down with either a mechanical roll (with speeds up to 10,000 m/min) or with air jets (with speeds to 30,000 m/min). If the air jets are placed in the melt die, this process is called melt blowing. The speeds possible with melt spinning are orders of magnitude higher than the speeds used in solution spinning. Hence, melt spinning is an inherently less expensive process for producing fibers.